CHENG, YING. Genetic Characterization of Genes Specific to Listeria monocytogenes
Epidemic-Associated Serotype 4b Strains (Under the direction of Dr. Kathariou Sophia.)
Listeria monocytogenes, a food-borne pathogen of humans and animals, can cause
severe listeriosis with relatively high mortality. A cluster of closely related strains of L.
monocytogenes (designated Epidemic Clone I) have been implicated in numerous outbreaks
in Europe and North America, including the California outbreak of 1985. L. monocytogenes
strains implicated in the 1998-1999 and the 2002 multistate outbreaks in the USA represent a
unique epidemic-associated clonal group, designated Epidemic Clone II (ECII). Comparative
genomic analyses across five genomes from different L. monocytogenes isolates and Listeria
species identified a genomic region (region-18) in serotype 4b strains that may have been
acquired by horizontal gene transfer. Region-18 is either absent or markedly divergent in
ECII strains but conserved among other serotype 4b strains. Region-18 is flanked by a large
gene encoding a putative cell-wall associated protein (wap) on one side and a well-known
virulence gene internalin A (inlA) on the other side in serotype 4b strains. PCR primers and
DNA probes derived from this ECII-specific region-18 can readily differentiate ECII strains
from other serotype 4b strains. This facilitates the detection and monitoring of these strains
belonging to ECII clonal group in foods, clinical samples, and the environment. Genetic
characterization of wap by the construction of deletion mutants suggested that the ECII wap
mutant but not the ECI wap mutant may be involved in specific environmental adaptations
such as surface adherence and possibly biofilm formation in ECII strains. Mutational and
functional analyses showed that the deletion mutant of region-18 in ECII had an enhanced
death rate during post-stationary incubation at 42?,suggesting that the ECII-specific region-
18 may be implicated in post-stationary stress responses. Two c72.44-negative variants of
epidemic-associated L. monocytogenes serotype 4b strains were isolated from laboratory
cultures. Naturally occurring c74.22-negative variants that exist under laboratory conditions
without any noticeable phenotypic differences from their original forms may complicate the
analysis of phage sensitivity and pathogenic characteristics.
Genetic Characterization of Genes Specific to Listeria
monocytogenes Epidemic-Associated Serotype 4b Strains
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